1. Field of the Disclosure
The present disclosure generally relates to electronic devices and, more particularly, to a system and method to reduce DQ pin capacitance in semiconductor memory chips.
2. Brief Description of Related Art
Memory devices are electronic devices that are widely used in many electronic products and computers to store data. A memory device is a semiconductor electronic device that includes a number of memory cells, each cell storing one bit of data. The data stored in the memory cells can be read during a read operation. FIG. 1 is a simplified block diagram showing a memory chip or memory device 12. The memory chip 12 may be part of a DIMM (dual in-line memory module) or a PCB (printed circuit board) containing many such memory chips (not shown in FIG. 1). The memory chip 12 may include a plurality of pins or balls 24 located outside of chip 12 for electrically connecting the chip 12 to other system devices. Some of those pins 24 may constitute memory address pins or address bus 17, data (DQ) pins or data bus 18, and control pins or control bus 19. It is evident that each of the reference numerals 17–19 designates more than one pin in the corresponding bus. Further, it is understood that the schematic in FIG. 1 is for illustration only. That is, the pin arrangement or configuration in a typical memory chip may not be in the form shown in FIG. 1.
A processor or memory controller (not shown) may communicate with the chip 12 and perform memory read/write operations. The processor and the memory chip 12 may communicate using address signals on the address lines or address bus 17, data signals on the data lines or data bus 18, and control signals (e.g., a row address select (RAS) signal, a column address select (CAS) signal, etc. (not shown)) on the control lines or control bus 19. The “width” (i.e., number of pins) of address, data and control buses may differ from one memory configuration to another.
Those of ordinary skill in the art will readily recognize that memory chip 12 of FIG. 1 is simplified to illustrate one embodiment of a memory chip and is not intended to be a detailed illustration of all of the features of a typical memory chip. Numerous peripheral devices or circuits may be typically provided along with the memory chip 12 for writing data to and reading data from the memory cells 26. However, these peripheral devices or circuits are not shown in FIG. 1 for the sake of clarity.
The memory chip 12 may include a plurality of memory cells 26 generally arranged in rows and columns to store data in rows and columns. A row decode circuit 28 and a column decode circuit 30 may select the rows and columns in the memory cells 26 in response to decoding an address provided on the address bus 17. Data to/from the memory cells 26 is then transferred over the data bus 18 via sense amplifiers and a data output path (not shown). A memory controller (not shown) may provide relevant control signals (not shown) on the control bus 19 to control data communication to and from the memory chip 12 via an I/O (input/output) circuit 32. The I/O circuit 32 may include a number of data output buffers or output drivers to receive the data bits from the memory cells 26 and provide those data bits or data signals to the corresponding data lines in the data bus 18. An exemplary I/O circuit is discussed below with reference to FIG. 2.
The memory controller (not shown) may determine the modes of operation of memory chip 12. Some examples of the input signals or control signals (not shown in FIG. 1) on the control bus 19 include an External Clock signal, a Chip Select signal, a Row Access Strobe signal, a Column Access Strobe signal, a Write Enable signal, etc. The memory chip 12 communicates to other devices connected thereto via the pins 24 on the chip 12. These pins, as mentioned before, may be connected to appropriate address, data and control lines to carry out data transfer (i.e., data transmission and reception) operations.
FIG. 2 is a simplified diagram illustrating a portion of the I/O circuit 32 in the memory chip 12 shown in FIG. 1. The I/O circuit 32 is shown to include an output driver unit 34 connected to the data (DQ) pins 18 of the memory chip 12. The driver 34 receives the data signals (DQ Out) 38 from the memory cells 26 to be output on the DQ pins 18 (e.g., during a memory read operation). Thus, the DQ Out signals 38 may be generated internally within the chip 12. The I/O unit 32 may further include an impedance calibration circuit 36 for the DQ output driver 34. The impedance calibration circuit 36 may be used to tune various transistors (not shown in FIG. 2) in the output driver 34 as discussed hereinbelow with reference to FIG. 3. In a DDR (Double Data Rate) DRAM (Dynamic Random Access Memory) memory chip, the output driver 34 may also include a set of ODT (On-Die Termination) legs or circuit portion 40 and a set of non-ODT legs or circuit portion 42, which are discussed hereinbelow with reference to FIG. 3.
The on-chip ODT circuit 40 may be used to improve signal integrity in the system. An ODT pin (one of the pins 24 on the chip 12) may be provided on the chip to receive an externally-supplied (e.g., by a memory controller (not shown)) ODT enable/disable signal to activate/deactive the ODT circuit 40. Although the ODT circuit 40 in FIG. 2 is shown connected to the DQ pins 18, in practice, corresponding ODT circuits 40 may be provided for any other pins on the chip 12 including, for example, the address pins 17 and the control pins 19. The ODT circuit 40 may be more prevalent in DDR SDRAMs (Synchronous Dynamic Random Access Memories). In operation, the ODT circuit 40 provides desired termination impedance to improve signal integrity by controlling reflected noise on the transfer line connecting the memory chip 12 to another processing device, e.g., a memory controller (not shown). In a DDR SDRAM, the termination register (not shown) that was conventionally mounted on a motherboard carrying memory chips is incorporated inside the DDR SDRAM chip to enable or disable the ODT circuit 40 when desired. The termination register may be programmed through the ODT pin (not shown) by an external processor (e.g., a memory controller) to enable/disable the ODT circuit 40. As is known in the art, for example, when two memory chips 12 are loaded in a system, then during a memory write operation to one of the chips 12, the ODT circuit 40 in the other chip (which is not receiving data) is activated to absorb any signal propagations or reflections received on the data lines 18 (or address or control lines) of that “inactive” chip. This selective activation/deactivation of the ODT circuit 40 (e.g., in the memory chip that is not currently sending or receiving data) prevents the “inactive” chip from receiving spurious signals, thereby avoiding data corruption in the chip. The ODT circuit 40 thus improves signal (e.g., data signals) integrity in the memory chip 12. The non-ODT circuit portion 42 in the output driver 34 may provide routine signal driver functions to data signals as is known in the art.
FIG. 3 illustrates an exemplary circuit layout for a portion of the I/O circuit 32 shown in FIG. 2. The DQ Out lines 38 shown in FIG. 2 are not included in the circuit configuration of FIG. 3 for ease of illustration and clarity. As is known in the art and as can be seen from FIG. 3, the DQ output driver impedance calibration circuit 36 is connected to the pairs of tuning transistors of the ODT legs 40 and non-ODT legs 42 in the output driver 34. Each of the ODT and non-ODT legs is connected to a respective DQ pin 18 as shown in FIG. 3. The ODT legs 40 as well as the non-ODT legs 42 of the output driver 34 provide necessary signal amplification and buffering to the data signals to be sent from the memory cells 26 to the DQ pins 18. However, the ODT legs 40 may additionally provide the ODT functionality when activated. Thus, although the ODT and non-ODT legs may be identically constructed (as shown, for example, in FIG. 3), in operation of the driver 34, the ODT legs 40 may provide output driver function as well as the ODT functionality, whereas the non-ODT legs 42 may just provide the data output driver function (data signal amplification and buffering). Each output of the driver 34 may have an IC (integrated circuit) output pad (not shown) to convey the data signals to the corresponding DQ pins 18 as is known in the art. It is noted here that only a portion of the output driver 34 is shown in FIG. 3 with constituent circuit details for ease of illustration and clarity. For example, only two (44 and 52) of the “i” (i>1) ODT legs 40 and two (60 and 68) of the “j” (j>1) non-ODT legs are shown in FIG. 3.
Additional circuit details of FIG. 3 are known in the art and, hence, are not discussed in detail here. It is observed, however, that it is desirable to devise an output driver circuit configuration that reduces capacitance on output (DQ) pins without affecting the speed with which signals may be output from the electronic device. It is further desirable to obtain such output driver mechanism without significantly adding logic circuitry on the chip real estate.